Microglial cells play a critical role in the inflammatory as well as degenerative processes of the central nervous system (CNS). Here we propose to specifically target the immune-relevant properties of microglial cells -without interfering with the systemic immune compartment- and to determine the effect of those changes on the development of MOG-EAE. Through the use of irradiation bone marrow (BM) chimeric mice, we will selectively alter the genotype of the microglial cells vs. the genotype of the hematopoietic compartment. This in vivo approach will provide compelling insights into the function of resident microglial cells in the development and progression of MOG-EAE. The susceptibility, pathogenesis and the development of MOG-EAE will be determined in mice where MHC II is absent in the major APCs in the CNS (microglia) or in the systemic APC, namely macrophages, DCs and B cells. These studies will resolve the contribution of microglial cells as APCs to the priming, migration and phenotype of encephalitogenic T cells as well as their contribution to disease development and progression. CD4O triggering has been shown to exert profound effects on microglial cells as well as on APC in the peripheral immune system. Further, blockade of CD4O signaling, even late in disease can ameliorate EAE. Using BM chimeric mice, the function of CD4O will be restricted to the CNS endogenous cells or the peripheral immune system. Such mice will allow us to clearly resolve the respective contribution of CD4O signaling within and outside of the CNS. More precise expression of class II MHC and CD4O will be obtained in mice by combining BM-chimerism with transgenicity. In these mice, we will restrict MHC II and CD4O expression to perivascular "microglia" or create an MHC II/CD4O-deficient perivascular compartment. CD4O expression by the peripheral immune system is essential for the development of EAE. Using a novel cohort of Tg mice that express Class II MHC or CD4O in specific APC (B cells or DCs or macrophages) we will determine the contribution of different, peripheral APCs to the development of actively induced or passively transferred MOG-EAE. These in vivo studies should clearly resolve how different APC populations, and the activation via CD4O in APCs controls the development of this T cell-mediated autoimmune disease. The need for CD4O signaling in APCs will be confirmed using mice in which a central CD4O signaling kinase, NIK, is mutated. Such mice are resistant to EAE induction and this kinase represents a new and exciting target for immune intervention in MS. To understand how CD4O regulates inflammation within the CNS, mice genetically deficient in IL-12 and IP-10 will be studied. To specifically assess the role and function of CNS-derived IP-lO and IL-l2, we will generate BM-chimeric mice in which the systemic immune compartment is fully competent, but the CNS unable to make these molecules. Together, these studies will help define the role of microglial cells as APCs during the priming and effector phase in EAE.